In which there is a secret warning of things in the far distant future but it's pretty bloody well hidden,
OR
In which Jareth will be OWED.
Erik belongs to Leroux and Webber. Jareth belongs to Henson, Lucas, and Bowie. Erlkönig is a mythological character and belongs to everyone and no one.
SARAH!!! IF YOU DON'T WANT TO GET RAPED AND DRAGGED BACK TO THE UNDERGROUND AS A BLOODY CONCUBINE THAN GET THE HECK OUTTA DODGE!!! AND BRING LOTS OF SALT!!!!!!!!! (I am a mythology enthusiast. This is beyond epic by the way. Now back to being worried for all the female characters!)
If his penchant for exacting loopholes, and executing clauses to the letter, is anything like Rumpelstiltskin's, I'm sure that Jareth is going to try and extort this deal with his father as much as he can.